Described below is a method, mobile station and base station for transmitting data in a mobile communications system. Particularly, the method is for transmitting codebook-based precoded data in a multiple antenna communications system.
A mobile communications system, particularly a wireless cellular system, is organised in form of neighbouring cells with each cell being served by a base station (BS). The base station provides communication facilities to and from mobile stations, i.e. user equipments (UE).
The Universal Mobile Telecommunications System (UMTS) is one of the third-generation (3G) mobile phone technologies. The long term evolution (LTE) of UMTS, i.e. the successor technology to 3G, also called fourth-generation technology (4G), is standardised in the 3rd Generation Partnership Project (3GPP).
One important aspect of LTE is precoding in multiple antenna communications systems, e.g. multiple input multiple output (MIMO) or multiple input single output (MISO) communications systems. In the following, the term MIMO includes variants like MISO or single input multiple output (SIMO) communications systems.
MIMO systems have become more and more important in recent years as they allow for increased channel capacity, higher reliability, and better range of these systems compared to single antenna systems. The main advantages of MIMO systems include a diversity gain, characterizing the system reliability with respect to fading, and a multiplexing gain, characterizing the system's capacity.
Precoding is used in transmitters to compensate for distortion introduced by the channel response and/or noise whitening filters used in the modem receivers.
Precoding denotes methods applied at a transmitter facilitating detection at a receiver. I.e., precoding is used in the transmitter in order to compensate for distortions on a communications channel.
As a drawback, precoding causes a feedback overhead which might become very high without careful design of the feedback link from user equipment to base station.
To overcome the drawback of a high feedback overhead, precoding based on codebooks was introduced. A codebook is a predefined set of precoding matrices with each precoding matrix being identified by a precoding matrix index. Therefore, a user equipment planning to establish a connection estimates the channel quality. Instead of sending the complete channel matrix back to the base station as feedback, the user equipment only selects and sends back the index of the best fitting precoding matrix out of a codebook.
As a further improvement, a tracking modus has been suggested for codebooks in 3GPP LTE. The tracking modus is useful in case of moving UEs or time varying radio channels as not the full codebook index has to be fed back from user equipment to BS. Instead, only a delta index is sent back from the UE. The delta index can be transmitted with fewer bits than the full index, as only a small part of the whole codebook will be a possible candidate for the next selection of a precoding matrix.
In case of beam forming—a special type of precoding where a signal processing technique is used to control the directionality of the reception or transmission of a signal—a codebook might define a regular set of beams within a given sector. When a user equipment is moving, only the adjacent beams are possible candidates for the next selection. In case of many antenna elements at the base station and a very accurate selection of the best beam, the codebook will have a large size. If the codebook size is reduced to minimize the feedback overhead, performance degradation due to sub-optimum precoding is the drawback.
Tracking allows reducing the feedback overhead, but if the codebook size is too small the achievable accuracy is limited.
For more advanced MIMO algorithms like joint transmission or non linear precoding where also interference between more than one beam occurs and where the transmission power as well as the beam angle has to be adapted very accurate, a small codebook size will lead to significant degradations while a large codebook size will increase the feedback overhead too much.
It is an aim of embodiments to address at the problem discussed above. One particular aim is to maximise precoding performance despite a limited codebook size.